1. Field of the Invention
The present invention relates to a semiconductor laser device.
2. Description of the Related Art
A conventional semiconductor laser device will be described with reference to FIGS. 6A through 6C.
FIGS. 6A and 6B are diagrams illustrating a front and face and a rear end face of a conventional semiconductor laser device 600, respectively. FIG. 6C is a diagram illustrating a cross-sectional view of the semiconductor laser device 600, as taken along the 6Cxe2x80x946C line indicated in FIGS. 6A and 6B.
As shown in FIGS. 6A and 6B, the semiconductor laser device 600 includes: a semiconductor substrate 1 made of an n-type InP material and having a mesa structure; a light confinement layer 2, provided on a mesa region of the substrate 1, made of an n-type InGaAsP (composition wavelength: about 1.05 xcexcm) and having a thickness of about 600 nm; an active layer 3 having a multiple quantum-well structure; a light confinement layer 4 made of a p-type InGaAsP material and having a thickness of about 600 nm; and a cladding layer 5 made of a p-type InP material and having a thickness of about 400 nm. The layers 2 through 5 are provided in this order on the semiconductor substrate 1. The active layer 3 includes seven InGaAsP well layers (not shown) each having a thickness of about 6 nm and a compressive distortion of 1.0% or less, and seven InGaAsP (composition wavelength: about 1.05 xcexcm) barrier layers (not shown) each having a thickness of about 10 nm and no compressive distortion, which are alternately layered on one another.
The semiconductor laser device 600 also includes: a first buried layer 6 made of a p-type InP whose carrier density is 7.0xc3x971017 cmxe2x88x923, a second buried layer 7 made of an n-type InP whose carrier density is 2.0xc3x971018 cmxe2x88x921, a third buried layer 8 made of an p-type InP whose carrier density is 7.0xc3x971017 cm3, and a contact layer 9 made of a p-type InGaAsP (composition wavelength: about 1.3 xcexcm). The layers 6 through 9 are provided in this order in the vicinity of the active layer 3 so as to surround the mesa region of the substrate 1.
In order to reduce the parasitic capacity in the buried layers 6 through 8 so as to improve the frequency response characteristic of the semiconductor laser device 600, grooves 14 are provided by using an etching technique. The grooves 14 extend into the first buried layer 6 via the contact layer 9, the third buried layer 8, and the second buried layer 7.
On the contact layer 9, a SiO2 film 10 is formed having a thickness of about 0.3 xcexcm with an aperture therein. A metal multilayer film 11 including three layers (i.e., an Au layer, a Zn layer, and an Au layer) is formed in the aperture, and a p-type electrode 12 is formed on the metal multilayer film 11.
An n-type electrode 13 is provided on the back side of the semiconductor substrate 1.
Referring to FIG. 6C, a cross-sectional view of the active layer 3 of the semiconductor laser device 600 is shown. The active layer 3 has a width of about 0.6 xcexcm within about 25 xcexcm from the front end face, while it has a width of about 1.6 xcexcm within about 25 xcexcm from the rear end face. The distance between the front end face and the rear end face is about 400 xcexcm, and the cross section of the active layer 3 has a stripe structure. The width of the active layer 3 having this stripe structure continuously decreases from the rear end face toward the front end face. Thus, the stripe width of the active layer 3 at the front end face is narrower than that at the rear end face. This is a structure of a semiconductor laser device for implementing a narrow output angle characteristics and a low operation current characteristics at a high temperature (Y. Inaba et al., IEEE JSTQE, vol. 3, 1399-1404, 1997). With this structure, the effect of confining light within the active layer 3 continuously decreases from the rear end face toward the front end face. Therefore, a large amount of light leaks out of the active layer 3 into the first and third buried layers 6 and 8) in the area adjacent to the front end face.
FIG. 7 is a graph illustrating the relationship between an operation environment temperature and an output angle of the semiconductor laser device 600. As seen from FIG. 7, when the operation environment temperature changes from about xe2x88x9240xc2x0 C. to about 85xc2x0 C., the output angle changes from about 14.0xc2x0 to about 10.2xc2x0 (i.e., about 3.8xc2x0). Therefore, in a case where light output from the semiconductor laser device 600 is coupled to an optical fiber 16 as shown in FIG. 8, for example, such temperature conditions change the coupling efficiency between the light from the semiconductor laser device 600 and the optical fiber 16, thereby changing the intensity of the light propagated through the optical fiber 16. This will adversely affect the transmission characteristics of the optical fiber 16. The amount of change in the intensity of the light propagated through the optical fiber 16 should satisfy the practical standard in optical communications (i.e., 1 dB or less for a temperature change for about xe2x88x9240xc2x0 C. to about 85xc2x0 C.). Otherwise, the transmission characteristics of the optical fiber would be very poor. Thus, the semiconductor laser device 600 may not be usable without optical components for focusing light (e.g., a lens) between the semiconductor laser device 600 and the optical fiber 16.
The conventional semiconductor laser device 600, however, does not satisfy the above-mentioned standard, and the amount of change in the intensity of the light propagated through the optical fiber is 2 dB.
According to one aspect of the present invention, there is provided a semiconductor laser device, including: a semiconductor substrate of a first conductivity type; an active layer having a stripe structure formed on the semiconductor substrate; a first buried layer of a second conductivity type formed on the semiconductor substrate and in a vicinity of the active layer; a second buried layer of the first conductivity type formed on the first buried layer and in the vicinity of the active layer; and a third buried layer of the second conductivity type formed on the second buried layer and in the vicinity of the active layer.
In this semiconductor laser device, a difference between a refractive index of the first buried layer and a refractive index of the second buried layer is about 0.02 or less, and a difference between the refractive index of the second buried layer and a refractive index of the third buried layer is about 0.02 or less.
According to the present invention, the difference between the refractive index of the first buried layer and that of the second buried layer, and the difference between the refractive index of the second buried layer and that of the third buried layer are set to be small, thereby reducing the amount of light propagating through the active layer which leaks into the first buried layer or the third buried layer.
In one embodiment of the present invention, the semiconductor laser device further includes a pair of light confinement layers formed so as to sandwich the active layer.
In another embodiment of the present invention, a cross section of the active layer at a front end face is smaller than that of the active layer at a rear end face.
In still another embodiment of the present invention, the semiconductor laser device further includes an optical fiber provided adjacent to the front end face of the active layer into which light output from the front end face of the active layer is input.
In still another embodiment of the present invention, a carrier density of the first buried layer and that of the third buried layer are substantially equal to each other.
In still another embodiment of the present invention, the first conductivity type is n-type and the following inequality is satisfied:
log10fn(x) less than 1.05 log10xxe2x88x921.47,
where x represents a carrier density of the second buried layer, and fn(x) represents a carrier density of the first buried layer and the third buried layer.
In still another embodiment of the present invention, the first conductivity type is p-type and the following inequality is satisfied:
log10fp(x) greater than 0.95 log10x+1.40,
where x represents a carrier density of the second buried layer, and fp(x) represents a carrier density of the first buried layer and the third buried layer.
Thus, the invention described herein makes possible the advantages of providing a semiconductor laser device in which the output angle is less dependent on the temperature even with a small cross section of the active layer at the front end face.
These and other advantages of the present invention will become apparent to those skilled in the art upon reading and understanding the following detailed description with reference to the accompanying figures.